If my memory serves me right, it's Python/Django. I contracted in that department a few years ago.

Edit: Just remembered it's all open source too! Look here and search for repos beginning "eq-". Survey runner is the app I believe. When I was there the plan was to use that, though perhaps they eventually wrote a fresh app. Anyway the code should all be there https://github.com/ONSdigital?q=Eq&type=&language=&sort=